Water vapor, facet erosion, and the degradation of (Al,Ga)As DH lasers operated at CW output powers of up to 3 mW/µ stripewidth

Abstract

Light versus current measurements on (Al, Ga)As DH lasers showed that output degradation is virtually absent in vacuum for lasers with little or no internal degradation, whereas it manifests itself strongly in water-rich ambients. The output degradation coincides with the presence of facet erosion. Moreover, a direct relationship has also been found between facet erosion and the occurrence of self-pulsations. In fact, mirror degradation has been shown to be a very dominant degradation mechanism for uncoated devices operated at output powers of 3 mW/μ stripewidth. Mirror coatings are of great help in reducing the degradation; however, the requirements imposed on the coating and the coating process turn out to be rather stringent. The aluminium content of the active layer plays a rather significant role in reducing the mirror related degradation, i.e., for lasers with wavelengths between 8200 and 8900 A. It is shown by means of an estimation that small distortions of the facet smoothness, which result from inhomogeneous mirror oxidation, are sufficient for producing a considerable increase of the threshold current. Vacuum testing of laser devices turns out to be a valuable tool for splitting up the overall degradation in external, i.e., mirror related, and internal parts. Moreover, it enables us to evaluate the merits of facet coatings. Experiments with lasers in dry nitrogen indicate that the water content below which facet erosion is absent is probably much less than 100 ppm.